Piezoelectric composite nanofibers, nanotubes, nanojunctions and nanotrees

ABSTRACT

Piezoelectric nanostructures, including nanofibers, nanotubes, nanojunctions and nanotrees, may be made of piezoelectric materials alone, or as composites of piezoelectric materials and electrically-conductive materials. Homogeneous or composite nanofibers and nanotubes may be fabricated by electrospinning. Homogeneous or composite nanotubes, nanojunctions and nanotrees may be fabricated by template-assisted processes in which colloidal suspensions and/or modified sol-gels of the desired materials are deposited sequentially into the pores of a template. The electrospinning or template-assisted fabrication methods may employ a modified sol-gel process for obtaining a perovskite phase in the piezoelectric material at a low annealing temperature.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Patent ApplicationNo. 60/957,034, filed Aug. 21, 2007, the entirety of which isincorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Some of the research performed in the development of the disclosedsubject matter was supported by grant CMMI #0826418 from the NationalScience Foundation. The U.S. government may have certain rights withrespect to this application.

FIELD OF THE INVENTION

The present invention relates to piezoelectric nanostructures, includinghighly-branched nanostructures (“nanotrees”), and variations of theaforesaid having multiple layers and/or electrically-conductivefeatures, and methods for fabrication of such nanostructures.

BACKGROUND OF THE INVENTION

Piezoelectric materials have been utilized widely in sensors andactuators. Compared to commonly used piezoelectric structures, such asthose based on bulk and thin films, piezoelectric fibers have attractedmore attention because they allow greater flexibility in the design andapplication of various structures. Such fibers can be made of a numberof materials, such as zinc oxide (ZnO), barium titanate (BaTiO₃), leadzirconate titanate (PbZr_(1-X)Ti_(x)O₃, PZT) or a piezoelectric polymersuch as polyvinylidine fluoride (PVDF). In particular, fibers made ofPZT have provided the basis for devices having high bandwidth, fastresponse, and high sensitivity.

While there are many methods for fabricating piezoelectric fibers havingmicroscale dimensions, there are few methods for fabricatingpiezoelectric nanofibers (i.e., fibers having dimensions on the order ofnanometers), including hydrothermal synthesis, sol electrophoresis andmetallo-organic decomposition (MOD) electrospinning. Fibers fabricatedby the hydrothermal and electrophoretic methods are discontinuous, whichlimits their usefulness as components of working devices. In contrast,the electrospinning method can fabricate continuous fibers havingdiameters from tens to hundreds of nanometers. Further, aligned fiberscan be fabricated using simple auxiliary methods.

Piezoelectric fibers, in general, have been used in active fibercomposites (AFC) as sensors and actuators. AFC typically comprisepiezoelectric fibers in a polymer matrix, and are more flexible androbust than monolithic piezoelectric devices because they combine thephysical properties of the fibers and the matrix. Devices known in theprior art have used fibers with diameters as small as 30 microns, butsuch fibers are too large to be embedded in active structures or microor nanoscale devices. Further, AFC typically incorporate interdigitatedelectrodes to simplify fabrication and take advantage of thenon-isotropic character of the piezoelectric properties of the fiber.

SUMMARY OF THE INVENTION

In one aspect, the subject matter disclosed herein is directed to thefabrication of piezoelectric structures having nanoscale dimensions, andthe characteristics and uses of the nanostructures themselves. Fibrousstructures (“nanofibers”), tubular structures (“nanotubes”), simplebranched structures (“nanojunctions”) and highly-branched structures(“nanotrees”) are disclosed. The disclosed nanostructures includestructures that are fabricated entirely from piezoelectric materials.The disclosed nanostructures further include composite nanostructureswhich comprise adjacent layers of piezoelectric materials andelectrically-conductive materials. Such composite nanostructures may actas mechanical-electrical energy transducers and as electrical conductorsor electrodes.

In another aspect, the subject matter disclosed herein is directed tomethods for fabricating homogeneous or composite nanofibers andnanotubes by electrospinning. The disclosed methods may employ amodified sol-gel process for obtaining a perovskite phase at a lowannealing temperature, which is also disclosed herein. The disclosedmethods also present methods and devices for aligning the nanofibers andnanotubes as they are collected. Devices are disclosed for fabricatingsingle homogeneous nanofibers and for fabricating multiple nanofibers athigh rates. Further, a co-axial device for electrospinning compositenanofibers and nanotubes, or homogeneous nanotubes, is disclosed.

In yet another aspect, the subject matter disclosed herein is directedto template-assisted methods for fabricating nanotubes, nanotubes andnanotrees, as homogeneous structures of piezoelectric materials orcomposite structures of piezoelectric materials andelectrically-conductive materials. In the disclosed methods, a templatehaving pores of the desired configuration (i.e., straight pores tofabricate nanotubes, simple branched pores to fabricate nanojunctions,or highly-branched pores to fabricate nanotrees) are selected orfabricated, and modified sol-gels or colloidal suspensions ofpiezoelectric materials or electrically-conductive materials aredeposited, sequentially, into the pores to build-up the desirednanostructure. The nanostructures may be solidified or annealed duringor after the build-up process. The disclosed template-assistedfabrication methods may also employ the modified sol-gel process thatwas discussed with respect to the disclosure of electrospinning methods.

In a further aspect, the subject matter disclosed herein is directed tomicron-scale active fiber composite devices comprising piezoelectricnanostructures (NAFC), and methods for fabricating such devices. Suchdevices comprise piezoelectric nanostructures that are in direct contactwith electrodes and encased in a dielectric matrix material. Suchdevices may, in an alternative, include composite piezoelectricnanostructures, which may eliminate the need to provide separateelectrodes in the NAFC.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, reference is madeto the following detailed description of the exemplary embodimentsconsidered in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a single-needle electrospinningapparatus used to fabricate homogeneous piezoelectric nanofibers.

FIG. 2 is a scanning electron microscopy (SEM) image of randomlydistributed piezoelectric nanofibers collected on a substrate.

FIG. 3 is a transmission electron microscopy (TEM) image of an annealedpiezoelectric nanofiber.

FIG. 4 is an x-ray diffraction pattern of an annealed nanofiberfabricated from a modified lead zirconate titanate (PZT) sol-gel.

FIG. 5 is a TEM image of an annealed PZT nanofiber showing crystallinestructures.

FIG. 6 is a SEM image of a PZT nanofiber collected across a trenchetched into a silicon substrate.

FIG. 7 is a schematic diagram of a three-point bending test performed onthe PZT nanofiber of FIG. 6 using an atomic force microscope (AFM).

FIG. 8 is a schematic diagram of a three-point bending test performed ona collection of annealed PZT nanofibers using a dynamic mechanicalanalyzer (DMA).

FIG. 9 is a reproduction of a screen-capture of the graphical outputobtained during the test of FIG. 8.

FIG. 10 is a schematic drawing of a multi-needle device for high-ratefabrication of multiple homogenous nanofibers.

FIGS. 11 a-11 h are schematic drawings representing steps in a processfor fabricating the device of FIG. 10.

FIG. 12 is a schematic drawing of a coaxial electrospinning device forfabrication of core-shell nanofibers.

FIGS. 13 a-13 c are schematic drawings representing steps in atemplate-assisted fabrication of a composite nanotube.

FIG. 14 is a schematic drawing of a set-up for vacuum-assistedfabrication of a nanotube.

FIG. 15 is a SEM image of a cross-section of an anodic aluminum oxidetemplate having PZT nanotubes within its pores.

FIG. 16 is a SEM image of a collection of annealed PZT nanotubes thathave been recovered from their template.

FIG. 17 is a SEM image of a transverse cross-section of the PZTnanotubes of FIG. 16.

FIG. 18 is an x-ray diffraction pattern of a PZT nanofiber of FIGS. 16and 17.

FIG. 19 is a schematic drawing of a nugget drop test performed onnanotubes within their template.

FIG. 20 is a graph of the voltage measured across a group of nanotubesduring the test of FIG. 19.

FIG. 21 is graph of the voltages measured across a group of nanotubesduring a series of nugget drop tests.

FIG. 22 is a SEM image of a simple Y-branched piezoelectricnanostructure.

FIG. 23 is a schematic drawing of a highly-branched piezoelectricnanostructure.

FIG. 24 is a graph of resonant frequencies modeled for the vibrationalmodes of a highly-branched piezoelectric nanostructure.

FIG. 25 is a schematic drawing of a template for the fabrication ofhighly-branched piezoelectric nanostructure.

FIG. 26 is a schematic drawing of a conventional active fiber compositedevice.

FIG. 27 is a schematic cross-sectional drawing of the active fibercomposite device.

FIG. 28 is a schematic cross-sectional drawing of an active fibercomposite device incorporating electrodes in direct contact withpiezoelectric nanofibers.

FIG. 29 is a schematic drawing of aligned nanofibers collected oninterdigitated electrodes over a dielectric substrate.

FIG. 30 is a SEM image of aligned nanofibers collected on a substrate.

FIG. 31 is a SEM image of a nanofiber active fiber composite beam.

DETAILED DESCRIPTION OF THE INVENTION Fabrication of PZT Nanofibers byElectrospinning Sol-Gel Precursors

As discussed herein, the fabrication of piezoelectric nanofibers (e.g.fibers having diameters with range of tens to hundreds of nanometers) byelectrospinning sol-gel precursors and annealing the collected fibers atlow temperatures is a promising technique for fabricating ceramicnanofibers, and other ceramic nanostructures, having excellentmechanical and piezoelectric properties. Nanofibers fabricated by suchmethods may be actuated in either transverse mode or longitudinal mode,which provides more flexibility in designing the devices and systems inwhich they are used. Such nanofibers may be used directly as sensors oras actuators in microscale or nanoscale devices. With appropriatesurface functionalization, nanofiber resonators could be used asbiosensors. Further, such nanofibers may be used as active structures inapplications for which thin films could not be used. Nanotubes (i.e.,hollow tubes having diameters in the same range as nanofibers) havingproperties and uses comparable to the aforesaid nanofibers may befabricated by a modified electrospinning process using aspecially-designed device discussed separately herein.

As an example of nanofiber fabrication, lead zirconate titanate (PZT)nanofibers (specifically, nanofibers formed from PbZr₅₂Ti₄₈O₃) wereprepared by an electrospinning process and tested to determine theirstructures and properties. The primary component of the precursormixture was a commercial PZT sol-gel (MMC Electronics America Inc.).Several materials were used to modify the viscosity and conductivity ofthe PZT sol-gel, from which poly(vinyl pyrrolidine) (PVP) was selectedfor further use. The PZT sol-gel was mixed with a solution of PVP inalcohol, and acetic acid was added to stabilize the solution and tocontrol hydrolysis of the sol-gel. The amount of PVP was varied tocontrol the diameter of the nanofibers. The compositions of theprecursor mixtures tested were as follows:

PZT sol-gel: 3 ml PVP: variable, but on the order of 0.1 to severalgrams Alcohol: 5 ml Acetic acid: 2 ml

It was observed that the diameters of nanofibers fabricated byelectrospinning could be controlled by adjusting the amount of PVP inthe precursor mixture. Continuous nanofibers having an average diameterof about 50 nm were obtained at a PVP concentration of roughly 0.007 gPVP/ml of precursor mixture. The average diameter of the continuousnanofibers was increased to about 150 nm by adjusting the PVPconcentration upward to roughly 0.028 g PVP/ml of precursor mixture. AtPVP concentrations above 0.028 g/ml, the diameters of the nanofibersdecreased gradually until continuous fibers could no longer becollected, probably due to the loss of PVP during the annealing process.

FIG. 1 is a schematic drawing of the electrospinning apparatus 10 usedto fabricate nanofibers for testing. The apparatus 10 includes astainless steel needle 12, which has an opening 14 with a diameter ofabout 200 microns, and which is mounted on a frame 16 over a collectingsubstrate 18. It is preferable to use a wafer of high-resistance siliconor a high-dielectric material such as alumina or silica as the substrate18. Parallel electrodes or interdigitated electrodes (not shown) may bedeposited on the substrate by any of a number of known methods to aid inaligning the nanofibers. Nanofibers collected for mechanical testing inthe present example were collected across 5 micron trenches etched intothe substrate.

In the electrospinning process, a feeding system 20, which includes ahigh-pressure pump (not shown) and syringe (not shown) containing theprecursor mixture, was used to provide a continuous supply of precursormixture to the needle 12. A high-voltage power source 22 was used toapply a voltage across the needle 12 and the substrate 18. Thehigh-pressure pump applied pressure to the syringe to maintain a flowrange of about 0.5 μl/min, which was sufficient to maintain a small dropof the precursor mixture at the needle tip in the absence of a voltageacross the needle 12 and the substrate 18. A high voltage (in thisinstance, 10,000 V) was then applied across the needle 12 and substrate18. The high voltage overcame the surface tension of drop at the tip ofthe needle, producing a highly-charged jet of the precursor mixture. Thejet underwent a stretching and whipping process during which the solventin the precursor mixture evaporated and nanoscale fibers were depositedon the collecting substrate.

The collected as-spun fibers (also, “green fibers”) were cured in athree-step process. First, the fibers were solidified at 80° C. for 5minutes, then held at 380° C. for 5 minutes to drive off solvent.Finally, the nanofibers were heated at 650° C. for 1 hour to create apure perovskite phase, in which phase PZT and other ceramic compoundshave piezoelectric properties. This temperature is lower than thatneeded to anneal nanofibers made from metal-organic precursors, possiblybecause of the precursor mixture that was used.

Annealed nanofibers fabricated as described above may be released fromthe substrate by any of a number of known dry etching methods, dependingon the substrate which is used. In instances where a nanofiber is laidacross a trench in a substrate, such release may not be required.

Test Results for PZT Nanofibers Fabricated by Electrospinning

Characterization: Nanofibers fabricated as described above werecharacterized by scanning electron microscopy (SEM), transmissionelectron microscopy (TEM) and X-ray diffraction. FIG. 2 is a SEM imageof a collection of randomly distributed PZT nanofibers 24 on a siliconsubstrate 26. Aligned nanofibers were obtained using pre-patternedelectrodes, such as those described above. FIG. 3 is a TEM image of aPZT nanofiber 28 having a diameter of about 150 nm.

FIG. 4 shows the x-ray diffraction pattern of the annealed PZTnanofibers. The pattern indicates that the nanofibers have a pureperovskite crystalline phase, which provides the piezoelectricproperties of the nanofibers. FIG. 5 is a TEM image of an annealed PZTnanofiber and shows that the crystalline grain sizes are about 10 nm.

Mechanical properties: The Young's modulus of a single nanofiber wasdetermined using a three-point bending test using atomic forcemicroscopy (AFM). FIG. 6 is a SEM image of a nanofiber 30 across a 5micron trench 32 etched into a silicon substrate 34, such as were usedin the test. FIG. 7 presents a schematic drawing of the test, in whichthe nanofiber 30 was deflected using the AFM tip 36. The tip 36 was usedto apply a force at the midpoint 38 of the suspended nanofiber 30. TheAFM was operated in contact mode, and a force plot was obtained todetermine the displacement of the suspended nanofiber and the appliedforce. The Young's modulus of the nanofiber was calculated to be 42.99GPa.Piezoelectric properties: The piezoelectric response of the nanofibersto strains in the transverse direction was evaluated using a three-pointbending test similar to that described above. FIG. 8 presents aschematic of the test. Aligned nanofibers 40 were collected on asubstrate comprising a titanium (Ti) strip 42 with a layer of zirconiumoxide 44 (ZrO₂) as an insulator. Conductive adhesive was used to attachthe nanofibers to the substrate and also as a pair of electrodes 46. Thedistance between the electrodes 46 was 1 mm. The three-point bendingtest was conducted using a dynamic mechanical analyzer (DMA) comprisingtwo fixed points 48, 50 in contact with the insulator 44 and a block 52to provide pressure to the opposite side 54 of the substrate 42.Different strains were applied to the substrate and the changes involtage across the electrodes were measured. A voltage of about 0.17 Vwas generated by applying 0.5% strain on the substrate. Thepiezoelectric coefficient, g₃₃, was calculated to be 0.079 V·m/N. FIG. 9is a reproduction of a screen-capture from a graphical display of thevoltage changes measured during the test. The higher peaks 56 of thegraph correspond to the application of the strain, while the smallerpeaks 58 correspond to the vibration of the substrate when the strainwas released.

Multi-Needle Device for Electrospinning Nanofibers

The single needle electrospinning process discussed above is notefficient in fabricating large numbers of nanofibers. FIG. 10illustrates a multi-needle spinning device 56 that has been designed forhigh rate fabrication of nanofibers.

Referring to the cross-section of the device 56 shown in FIG. 10, thedevice 56 comprises an element 58 having a low electrical resistance,such as a wafer of doped silicon or a metal plate, and a dielectricelement 60 that is joined to the low-resistance element 58. Thedielectric element has a concave recess 62 that faces the low-resistanceelement 58 so as to form a chamber 64. The chamber 64 is hydraulicallyconnected to a high-pressure pump (not shown) through tube 66 by meansof fitting 68, which communicates with chamber 64 through port 70. Thelow-resistance element 58 has a number of hollow needles 72, each havinga low electrical resistance, that penetrate the low-resistance element58 through holes 74 so that they are hydraulically connected to thechamber 64.

The device is intended to replace the needle 12 of the electrospinningapparatus 10 illustrated by FIG. 1. Referring again to FIG. 10, afeeding system (not shown) comprising a syringe and a high-pressure pumpwould be provided to continuously supply the precursor mixture to all ofthe needles 72 through the chamber 64. A high voltage would be appliedacross the low-resistance element and a collecting substrate (notshown), causing nanofibers to be spun from all needles 72simultaneously.

A fabrication process for the multi-needle device is illustrated incross-section in FIGS. 11 a-11 h. A low-resistance wafer or plate(subsequently, “the low-resistance wafer” 76), corresponding tolow-resistance element 58 of FIG. 10, is provided (FIG. 11 a), and asilicon-dioxide mask 78 is laid on its surface 80 (FIG. 11 b). Openings82 in the mask correspond to the outer diameters and desired locationsof the holes 84 corresponding to holes 74 in the completed device 56(see FIG. 10). Referring again to FIGS. 11 a-11 h, the low-resistancewafer 76 is then etched by deep reactive ion etching (RIE) to createholes 84 that penetrate through it in the positions of the openings 82in the silicon dioxide mask 78 (FIG. 11 c). The mask 78 is then removedfrom the low-resistance wafer 76 by buffered-oxide etching (BOE) (FIG.11 d).

A dielectric wafer (subsequently, “the dielectric wafer” 86) is provided(FIG. 11 e) and a silicon dioxide mask (not shown) is laid on itssurface 88 to protect an area 90 (shown in cross-sectional side view),extending along its perimeter. A concave recess 92 is then etched intothe dielectric wafer 86 by RIE, but not through the wafer 86, so as toleave a thickness 94 of the wafer 86 intact (FIG. 11 f). A port 96 isthen etched through the remaining thickness 94 of the dielectric wafer86 to receive a fitting (not shown) that will provide a hydraulicconnection between the recess 92 and a high-pressure pump (not shown)(FIG. 11 g). The dielectric wafer 86 is then wafer-bonded to thelow-resistance wafer 76 with the recess 92 facing the low-resistancewafer 76 so as to form a cavity 98 (FIG. 11 h). Needles (not shown),corresponding to needles 72 of FIG. 10, are then fitted to thelow-resistance wafer 76 through holes 84, and a fitting (not shown)corresponding to fitting 68 of FIG. 10 is inserted into the dielectricwafer 86 through port 96 to produce a device such as device 56 shown inFIG. 10.

Core-Shell Composite Nanofibers

Core-shell type composite nanofibers or nanotubes, comprising discretelayers of piezoelectric materials and electrically-conductive materials,may be formed by electrospinning or by templating methods. Structures ofcomposite nanofibers may include, for example, those in which a solidfiber of an electrically-conductive material is provided with an outerlayer of a piezoelectric material, or in which a layer of electricallyconductive material overlies a layer of piezoelectric material, whichmay also overlie an electrically-conductive core. Structures ofcomposite nanofibers may include, for example, those in which a tube ofpiezoelectric material has an inner layer of an electrically-conductivematerial, or both inner and outer layers of an electrically-conductivematerial. Such composite nanofibers or nanotubes may be used in place ofhomogenous nanofibers in the applications discussed above, and, becauseof their electrically-conductive properties, may eliminate the need forthe interdigitated electrodes used in active fiber composites (AFC),which are discussed separately.

The piezoelectric material discussed herein is PZT, but otherpiezoelectric materials, such as ZnO, BaTO₃, or a piezoelectric polymerlike PVDF, may be used. The electrically-conductive material discussedherein is indium titanium oxide (ITO), but other electrically-conductivematerials, such as other electrically-conductive metallic compounds(e.g., cadmium sulfide (CdS)), noble metals (e.g., gold or platinum), orelectrically-conductive polymers, may be used.

Fabrication of Composite Nanofibers by Electrospinning

Electrospinning can be used to fabricate core-shell composite nanofibersusing any piezoelectric and electrically-conductive materials that canbe prepared in sol-gel forms or nanoparticle colloid forms. Theelectrospinning methods used to prepare composite nanofibers are similarto those described above with respect to fabrication of homogenousnanofibers, except as modified to use two precursor mixtures, ratherthan one. Further, piezoelectric nanotubes withoutelectrically-conductive layers (i.e., homogeneous nanotubes) may also bemade using a further modification of the method described herein.

FIG. 12 is a schematic drawing of a coaxial electrospinning device 100for the fabrication of core-shell nanofibers, such as those describedabove. The device 100 comprises a tube housing 102 for receiving firstand second Teflon® tubes 104, 106, which are hydraulically connected,respectively, to first and second syringes (not shown). A stainlesssteel tube 108 is hydraulically connected to the first syringe throughfirst tube 104. A fused silica tube 110 is situated concentricallywithin stainless steel tube 108, and is sized so that there is aconcentric gap (not shown) between stainless steel tube 108 and fusedsilica tube 110. The fused silica tube 110 is hydraulically connected tothe second syringe through second tube 106. The stainless steel tube 108and fused silica tube 110 have respective open tips 112, 114 outside ofthe tube housing 102. In one embodiment of the device 100, the stainlesssteel tube 108 has an inner diameter of 460 microns and the fused silicatube 110 has an outer diameter of 360 microns and an inner diameter of75 microns. A first Teflon® sleeve 116 holds stainless steel tube 108 inplace and acts as a seal between the stainless steel tube 108 and thetube housing 102. A second Teflon® seal 118 holds fused silica tube 110in place and acts as a seal between first tube 104 and second tube 106.Persons skilled in the relevant arts will recognize that materials otherthan stainless steel and fused silica may be used for tubes 108, 110,and that materials other than Teflon® may be used for the other tubes104, 106 and seals 116, 118. Persons skilled in the relevant arts willalso recognize that the design of the device 100 can be modified toaccommodate three or more syringes.

In one embodiment of the electrospinning process, the first syringe isfilled with a PZT precursor mixture and the second syringe is filledwith an ITO precursor mixture. It will be recognized from theconfiguration of the device 100 shown in FIG. 12 that the contents ofthe first syringe will be extruded through the gap between stainlesssteel tube 108 and fused silica tube 110 to form an outer layer of thenanofiber, and that the contents of the second syringe will be extrudedthrough the open tip 114 of fused silica tube 110, to form an inner coreof the nanofiber. It will be further recognized that providing a PZTprecursor material through first tube 104 without providing any materialthrough second tube 106 will result in the fabrication of a PZTnanotube.

The electrospinning apparatus for producing core-shell nanofibers may beconfigured according to the schematic in FIG. 1, with the device 100 ofFIG. 12 replacing needle 12 of FIG. 1. Further, separate high-pressurepumps (not shown) should be provided to apply pressure to the respectivesyringes and maintain a continuous flow of the precursor materials totips 112 and 114. In other respects, the electrospinning process forproducing core-shell nanofibers may be the same as the processpreviously described for producing homogenous nanofibers. The annealingand release processes may also be the same as described for thefabrication of homogenous nanofibers. If a third, outer layer ofmaterial is desired, it may be applied by coating, chemical vapordeposition, or other known processes suitable for the material to beused, or it may be co-extruded through a tip adapted from thatillustrated in FIG. 12.

Fabrication of Composite Nanofibers and Composite or HomogenousNanotubes by Template-Assisted Processes

Composite nanofibers and nanotubes may be produced by depositing layersof precursor mixtures for piezoelectric materials and precursors forelectrically-conductive materials within the pores of a dielectrictemplate and annealing the nanofibers or nanotubes within the template.Homogeneous nanotubes may also be prepared by a similar depositionmethod, where all of the deposited layers comprise a precursor mixturefor a piezoelectric material. Template-assisted methods can be used tofabricate nanofibers and nanotubes, or other nanoscale piezoelectricstructures, such as those discussed elsewhere in this specification,using any pairs of piezoelectric and electrically-conductive materialswhich can be prepared as solutions, sol-gels or nanoparticle colloids,or as vapors such as those used in chemical vapor deposition. Further,although the exemplary methods discussed herein use anodic aluminumoxides (AAO), persons skilled in the relevant arts will recognize thatother materials, including other ceramic materials or silicon, can beused to form useful templates.

FIGS. 13 a-13 c illustrate a generalized procedure for template-assistedformation of composite, nanofibers or composite or hetrogeneousnanotubes. First, an appropriate template 120 is selected that has pores122 having sizes commensurate with the desired outer diameter of thenanostructure to be fabricated. These pores 122 will extend through theentire thickness of the template 120. AAO templates havingsubstantially-aligned pores of known sizes can prepared on aluminum foilby known methods using the foil as a support layer for the template.Suitable template materials are also available from commercial sources,or templates having desired thicknesses, pore diameters and porestructures may be custom-made. In the example shown in FIG. 13 a,precursor for a piezoelectric material, in liquid or vapor form, isdeposited, coated or grown on the interior surfaces of the pore 122 toform a shell layer 124. In a variation of the general process, aprecursor for an electrically-conductive material may be used in placeof the precursor for the piezoelectric material. The thickness of theshell 124 can be controlled by progressively adding layers of thedesired material.

Turning to FIG. 13 b, when a shell having the desired thickness has beenformed, a second shell 126, or a core (not shown), of a precursormixture for a second material can be formed on the interior of the firstshell 124 by adding layers of the appropriate precursor until a secondshell 126 of a desired thickness, or a core, has been formed. Thisprocess can be adapted to produce at least a third shell (not shown), ifdesired. The resulting nanostructure is then annealed to producenanofibers or nanotubes having the desired piezoelectric and conductiveproperties. Turning to FIG. 13 c, the template may then be etched awayto recover the nanofibers or nanotubes (e.g., composite nanotube 128).Because the pores in the templates will be substantially aligned, therecovered nanostructures will also be substantially aligned.

Fabrication and Testing of PZT Nanotubes

In an example of the generalized procedure described above with respectto FIGS. 13 a-13 c, nanotubes comprising a shell of PZT were formed inthe pores of an MO template using a vacuum-assisted deposition process.FIG. 14 is a schematic illustration of the set-up for thevacuum-assisted deposition process. An MO template, 130, having pores132, was placed on a filter holder 134 connected to a vacuum pump (notshown) by a tube 136. A PZT precursor mixture 138 was then placed on thetemplate 130, and the vacuum pump was allowed to run until about half ofthe precursor mixture 138 had been pulled into the pores 132. Excessprecursor mixture 138 was removed from the outside of the template 130using acetic acid, and the template 130, with precursor mixture 138within its pores 132, was dried at 80° C. to remove solvent and solidifythe precursor mixture 138 to form nanotubes (not shown). The nanotubeswere then annealed by heating at 380° C. for 5 minutes, then at 650° C.for 1 hour to create a pure perovskite phase of the PZT.

Characterization of PZT nanotubes: Nanotubes fabricated as describedabove were characterized by scanning electron microscopy (SEM),transmission electron microscopy (TEM) and X-ray diffraction. FIG. 15 isa SEM image of a cross-section of an MO template 140 before thenanotubes 142 are collected. It can be seen that the nanotubes 142 (darkstrips) are substantially aligned. FIG. 16 is a SEM image of acollection of substantially-aligned PZT nanotubes 144 that have beenrecovered from the MO template (not shown) in which they were formed.These tubes have diameters of about 190 nm to 210 nm and wallthicknesses of about 20 nm. They were formed with five layers, byconsecutive deposition steps. FIG. 17 is an end-view of the PZTnanotubes 144 of FIG. 16. The templates used to form the nanotubes 144were nominally 60 microns thick, resulting in nanotubes that werenominally 60 microns long.

FIG. 18 shows the x-ray diffraction pattern of the annealed PZTnanotubes 144. The pattern indicates that the nanotubes 144 have a pureperovskite crystalline phase, which provides the piezoelectricproperties of the nanotubes 144.

Electromechanical coupling tests: Nugget drop tests were conducted onPZT nanotubes within an AAO template to demonstrate the piezoelectricproperties of the nanotubes. AFM and DMA tests, similar to thosediscussed above with respect to PZT nanofibers, can also be performed onPZT nanotubes with little to no modification of the procedurespreviously discussed. Other tests typically used to evaluate mechanicaland piezoelectric properties of materials, such as dynamic vibrationtests, may also be performed.

FIG. 19 is a schematic illustration of the nugget drop test. Upper andlower electrodes 146, 148 were formed on the opposite faces 150, 152 ofthe AAO template 154 using a conventional technique, so as to contactthe ends 156, 158 of the nanotubes 160 within the template. Nuggets 162were dropped from different heights onto the upper electrode 146. Theimpact force of the nugget 162 onto the electrode 146 was transferred tothe nanotubes 160, resulting in their deformation and an accumulation ofcharge upon them.

FIG. 20 is a graph of the voltage measured during a typical drop. As canbe seen, the impact of the nugget causes a sudden spike in the voltagemeasured across the template. A sudden reversal of voltage also occursimmediately after the spike, probably resulting from rebound of thedeformed material.

FIG. 21 is a graph of the measured voltage as a function of the heightfrom which the nugget was dropped. The measured voltage increases in aroughly linear relationship with the drop height.

Branched Piezoelectric Structures

FIG. 22 is a SEM image of a simple Y-branched piezoelectric structure164 (“nanojunction”) in a nanofiber formed by a template-assistedmethod. Such structures 164 occur when the pores in the template arethemselves branched. Branched pores are often unintended artifacts ofthe template formation process. However, branched pores, includinghighly-branched pores, can intentionally be formed in templates by acontrolled process described herein, and used to form highly-branchedpiezoelectric nanostructures (“nanotrees”). An example of such ananotree 166 is illustrated schematically in FIG. 23. As can be seen, ananotree has a primary stem 168 and one or more generations of branches170, 172 174 connected to the stem 168. The nanotree 166 may be attachedthrough its stem 168 to an insulating oxide layer 176 or other substrate178. Such nanotrees may be fabricated with layers of piezoelectric andelectrically-conductive materials, or may be homogenously formed from apiezoelectric material.

Because of their highly-branched structures, piezoelectric nanotrees,such as nanotree 166, have potential applications in energy scavengingor as high-frequency wide-band energy filters. In general, apiezoelectric structure can convert mechanical vibrational energy intoelectrical energy, with the conversion efficiency peaking at thestructure's resonant frequency. Finite element analysis of a nanotreestructure showed that it has a number of complex vibrational modes,resulting in a series of closely-spaced resonant frequencies (FIG. 24).As a result, a nanotree structure can be used to scavenge energy from awide range of ambient mechanical vibrations having the same frequencies.A nanotree structure can also be used as a high-frequency wide-bandenergy filter that passes signals at frequencies other than its resonantfrequencies. Because a nanotree structure has numerous closely-spacedresonant frequencies, the filter is “wide-band” in nature.

Fabrication of Templates Having Highly-Branched Pores

The fabrication of templates having highly-branched pores is discussedherein with respect to MO templates. Methods for fabricating suchtemplates from other materials, such as other ceramic materials, will berecognized by persons having skill in the relevant arts.

Typically, a porous MO template may be formed by an electrolytic processwherein a direct current is passed through an acidic electrolyte, usingan aluminum foil as the anode on which the MO is formed. Pore size iscontrolled by balancing the rate at which MO is formed with the rate atwhich it is etched by the acid.

FIG. 25 is a schematic representation of an MO template 180 havinghighly-branched pores 182 for fabricating piezoelectric nanotrees. In amethod for fabricating templates, such as template 180, the anodizationprocess is started according to the typical method for producing MOtemplates having pores without branches. When primary pores 184 haveformed, the anodizing voltage will be reduced to (½)^(1/2) of itsinitial value causing the primary pore 184 to branch in a Y-formation,forming secondary pores 186. Additional generations of Y-branched pores188, 190 can be formed by further sequential reduction of the anodizingvoltage. In fact, highly-branched pore structures can be obtained byadjusting the anodizing voltage to (1/n)^(1/2) of its initial value,where n is the number of generations of pores. For example, branches 190would represent the fourth generation of pores, and would be formed byadjusting the anodizing voltage to (¼)^(1/4) of its initial value.

Fabrication of Composite or Homogeneous Nanotrees

Composite nanotrees may be produced using adaptations of thetemplate-assisted methods discussed with respect to composite orhomogenous nanofibers and nanotubes. That is, composite nanotrees may beformed in templates having highly-branched pores by forming layers ofprecursor mixtures for piezoelectric materials and precursor mixturesfor electrically-conductive materials within the pores and annealing thenanotrees within the template. Homogeneous nanotrees may also beprepared by a similar layer formation process, wherein all of thedeposited layers comprise a precursor mixture for a piezoelectricmaterial. Composite nanotrees can be fabricated from any pairs ofpiezoelectric and electrically-conductive materials which can beprepared as solutions, sol-gels, or nanoparticle colloids, or as vapors,such as those used in chemical vapor deposition.

Characterization and Testing of Nanotrees

Nanotrees may be characterized and tested by the same methods used tocharacterize and test nanofibers or nanotubes. For example, nanotreesmay be characterized using SEM, TEM and X-ray diffraction. Mechanicaland electromechanical coupling tests that may be used include AFM andDMA tests, dynamic vibration tests, and nugget drop tests.

Nanoscale Active Fiber Composites (NAFC)

FIGS. 26 and 27 are schematic illustrations, in orthogonal andcross-sectional views, respectively, of a typical design for an activefiber composite (AFC) device 192, wherein piezoelectric fibers 194 areencased in a dielectric matrix 196, such as a resin epoxy, onto whichelectrodes 198, 200 are deposited. As a result, there areelectrically-insulated gaps 202, 204 between the electrodes 198, 200 andthe fibers 194. When a voltage is applied across the electrodes 198, 200of such an AFC device 192, most of the voltage drop will occur acrossthe insulated gap 202, 204, making it necessary to supply a highvoltage, sometimes in the thousands of volts, to actuate the AFC device192. At such voltages, the dielectric matrix 196 may break down,diminishing the efficiency of the AFC device 192. FIG. 28 is across-sectional schematic view of a NAFC device 206 fabricated usingpiezoelectric nanofibers 208. The nanofibers 208 are coated withdielectric matrix material 210, with electrodes 212, 214 depositeddirectly on the nanofibers 208. Because the electrodes 212, 224 are indirect contact with the nanofibers 208, the NAFC 206 can be actuated atvoltages as low as a few volts to a few tens of volts.

Fabrication of NAFC Devices

In a method of fabricating NAFC devices, aligned piezoelectricnanofibers are collected on a substrate during an electrospinningprocess. As discussed elswhere with respect to electrospinning, shortnanofibers may be collected on dielectric layers over a grounded,uniformly-deposited electrode or doped silicon wafer, or a trench can bemade over the electrode, and the short fibers collected across thetrench. Referring to FIG. 29, interdigited electrodes 216, 218 can beformed on a silicon wafer 220 and electrically grounded, and long fibers222 collected across the electrode fingers 224, 226. The alignment ofthe nanofibers is controlled by the electric field applied to thesubstrate during the electrospinning process. These methods can bereadily adapted for collection of aligned composite nanofibers. Acollection of aligned nanofibers 228 is shown in FIG. 30.

After the aligned nanofibers have been collected, electrodes (not shown)may be deposited directly on top of the nanofibers by methods such assputtering or e-beam deposition. Then a matrix material, such a resinepoxy or silicone in solvent, may be deposited on the electrodes bymethods such as spin-coating, and the solvent baked off in a curingprocess. By adjusting the ratio of solvent to silicone, matrix membraneshaving thicknesses as small as about 1.0 micron to about 1.2 micronshave been obtained.

Testing of NAFC Devices

An example of a NAFC beam 230 made by a fabrication process such as thatdescribed is shown in FIG. 31. The beam is about 3 to 4 microns wide andextends over a 20 micron trench 232 in a silicon substrate. The ends ofthe beam are anchored to the substrate.

The mechanical properties of the beam were tested using an AFM methodsimilar to the three-point deflection test discussed above with respectto testing a homogenous nanofiber. The stiffness of the beam wascalculated to be 0.148 N/m.

Other methods of testing piezoelectric structures, such as the DMAtests, dynamic vibration tests, and nugget drop tests discussedelsewhere in this specification, may be adapted for testing theproperties of NAFC devices. Appropriate adaptations will be recognizedby persons skilled in the relevant arts.

It should be understood that the embodiments described herein are merelyexemplary and that a person skilled in the art may make many variationsand modifications thereto without departing from the spirit and scope ofthe present invention. All such variations and modifications, includingthose discussed above, are intended to be included within the scope ofthe invention, which is described, in part, in the claims presentedbelow.

1. A method of preparing a nanoscale piezoelectric structure, comprisingthe steps of: providing a template having a pore extending therethrough,said pore having an interior surface; depositing a precursor materialinto said pore such that said precursor material remains in said pore indirect or indirect contact with said interior surface, said precursormaterial being a material that is transformable to a piezoelectricmaterial upon heating; and heating said precursor material, whereby saidprecursor material is transformed into a piezoelectric material.
 2. Themethod of claim 1, wherein said depositing step comprising the step offorming a layer of said precursor material on said interior surface. 3.The method of claim 2, comprising the further steps of solidifying saidlayer of said precursor material, and depositing a second material ontosaid precursor material, wherein said second material comprises either amaterial that is electrically-conductive in a solid form or a materialthat is transformable into a piezoelectric material upon heating.
 4. Themethod of claim 1, comprising a further step of depositing anothermaterial into said pore, thereby forming a layer of said anothermaterial on said interior surface, said another material including amaterial that is electrically-conductive in a solid form, wherein saidstep of depositing said another material is performed before said stepof depositing said precursor material, and said step of depositing saidanother material includes the step of depositing said precursor materialonto said another material.
 5. The method of claim 4, further comprisingthe step of if said layer of said first material is not a solid layer,then solidifying said another material before said step of depositingsaid precursor material.
 6. The method of claim 2, comprising thefurther step of dissolving at least a portion of said template, wherebyat least a portion of said piezoelectric material is exposed.
 7. Themethod of claim 6, comprising the further step of depositing anelectrically-conductive material on said at least a portion of saidpiezoelectric material.
 8. The method of claim 1, wherein said pore is abranched pore.
 9. The method of claim 8, wherein said branched poreincludes branches having further branches extending therefrom.
 10. Themethod of claim 1, wherein said precursor material comprises leadzirconate titanate, and said heating step includes the step ofmaintaining said material at a temperature of about 650° C. until atleast a portion of said lead zirconate titanate is transformed intocrystals having a perovskite structure.
 11. The method of claim 1,wherein said precursor material further includes at least one organicliquid, and said heating step includes the further step removingsubstantially all of said at least one organic liquid from saidprecursor material before said maintaining step.
 12. The method of claim1, wherein said precursor material comprises lead zirconate titanate,poly(vinyl pyrrolodine) and an alcohol, and said heating step includesthe steps of maintaining said precursor material at 380° C. for at least5 minutes, then maintaining said material at a temperature of about 650°C. for at least about one hour.
 13. The method of claim 1, wherein saidprecursor material is a fluid in the form of a sol-gel or ananocolloidal suspension.
 14. The method of claim 13, wherein said porehas an opening through a side of said template and another openingthrough another side of said template, and said depositing step includesthe steps of placing a portion of said precursor material onto saidopening and applying a vacuum to said pore at said another opening,thereby drawing a portion of said precursor material into said pore. 15.The method of claim 3, wherein said material that is conductive in itssolid form comprises indium titanium oxide.
 16. The method of claim 7,comprising the further step of depositing a dielectric material on atleast a portion of said electrically-conductive material and said atleast a portion of said piezoelectrical material.
 17. The method ofclaim 1, wherein said template comprises an anodized aluminum oxide. 18.A method of preparing a nanoscale piezoelectric structure, comprisingthe steps of: providing a template having a pore extending therethrough,said pore having an interior surface; depositing a first material intosaid pore, thereby forming a layer of said first material on saidinterior surface of said pore, said layer having an exposed surfacefacing away from said interior surface of said pore; depositing a secondmaterial into said pore, thereby forming a layer of said second materialon said exposed surface, wherein at least one of said first and secondmaterials is a material that is transformable to a piezoelectricmaterial upon heating; and heating said first and second materials,whereby said at least one of said materials is transformed into apiezoelectric material.
 19. A method of fabricating piezoelectric fibershaving nanoscale diameters, comprising the steps of: providing anelectrospinning apparatus comprising a hollow, electrically-conductiveneedle having an end with an opening having a micron-scale diameter, anopposite end with another opening, a high-resistance substrate opposedto said opening, and a voltage means for providing a high voltage acrosssaid needle and said substrate; selecting a precursor materialcomprising lead zirconate titanate, poly(vinyl pyrrolodine) and analcohol, said precursor material being a sol-gel of said lead zirconatetitanate; providing said precursor material to said another opening ofsaid needle at a controlled rate while providing a high voltage acrosssaid needle and said substrate, so as to spin a fiber therefrom;collecting said fiber on said substrate; and heating said fiber at atemperature of about 80° C. for about 5 minutes, then heating said fiberat a temperature of about 380° C. for about 5 minutes, then maintainingsaid fiber at a temperature of about 650° C. for about 1 hour, therebytransforming at least a portion of said lead zirconate titanate intocrystals having a perovskite structure.
 20. An article, comprising ananoscale piezoelectric structure including a branched structure havinga stem with a plurality of branches extending therefrom and a furtherplurality of branches extending from at least one of said plurality ofbranches, wherein said stem, said at least one of said plurality ofbranches and at least one of said further plurality of branchescomprises a piezoelectric material.